arm64: dts: hikey960: Update HiKey960 with GPIO line names

This adds line names for all the GPIOs I could identify on the HiKey960
schematic.

"GPIO-A" through "GPIO-L" are the most important since they give users
a handle to look up the standard 96boards GPIOs from the GPIO character
device.

The rest of the names are more informational, nice debug information
for "lsgpio" so you can see that the right line is taken for the right
function in the kernel for example.

Cc: Wei Xu <xuwei5@hisilicon.com>
Cc: Zhangfei Gao <zhangfei.gao@hisilicon.com>
Cc: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>